Dialkyldithiophosphoric acid formaldehyde condensation products



Patented Dec. 11, 1951 DIALKYLDITHIOPHOSPHORIC ACID FORM- ALDEHYDE CONDENSA'IION PRODUCTS Edwin 0. Hook, Old Greenwich, Conn., and Philip H. Moss, Seldovia, Territory of Alaska, assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Original application March 26, 1948, Serial No. 17,394.- Divided and this application August 18, 1949, Serial No. 111,082

2 Claims.

This invention relates to the production of oilsoluble antioxidants for mineral and vegetable oils including lubricating oils, cutting oils, slushphoric acids have heretofore been employed in mineral lubricating oils and for other purposes,

and are known to possess antioxidant proper-- ties. In most cases these compounds have been used in the form of metal salts; usually as salts of polyvalent metals. Ordinarily the dialkyldithiophosphoric acids are prepared by reacting approximately 4 mols of a monohydric aliphatic alcohol such as ethanol, butanol, decanol, or octadecanol with 1 mol of P235. Typical methods of preparation are described in U. S. Patents Nos. 2,364,284, 2,228,658 and Reissue No. 22 ,829, which patents also describe the formation of heavy metal salts from these acids and the preparation of compounded lubrication oils from the neutral salts.

There is a growing demand in the lubricating oil industry for non-metallic oxidation inhibitors for lubricating oils, which may be employed in relatively small quantities to improve the grade of automobile engine lubricants such as solventrefined Mid-Continent stock and oils from Pennsylvania crudes. commonly used either alone in the oil or in admixture with detergents or with combination inhibitors and detergents such as the polyvalent metal salts of dialkyl-, dicycloalkylor diaryaldithiophosphoric acids. The present invention is of particular importance in the production of oilsoluble antioxidants which contain .a high percentage of combined phosphorus and sulfur, and which therefore possess good antioxidant properties, but which cause little or no increase in the corrosiveness of the oil.

We have found that a new class of lubricating oil antioxidants possessing the above and other important properties are obtained by condensing an 0,0-dialkyldithiophosphoric acid diester with formaldehyde or with a formaldehydeyielding substance such as paraformaldehyde, trioxymethylene and the like. These condensation products are soluble in lubricating oils and other hydrocarbon oils, and also in vegetable These oxidation inhibitors are glycerlde oils, and aretherefore well-suited for accomplishing the objects of the invention.

The condensation between the formaldehyde or formaldehyde-yielding substance and dialkyldithiophosphoric acid takes place readily at ordinary atmospheric temperatures. The reaction may be carried out simply by mixing an 0,0-dialkyldithiophosphoric acid with an aqueous formaldehyde solution such as ordinary commercial 37% formalin, or it may be carried out in the presence of an inert solvent such as benzene, toluene, aromatic hydrocarbons of higher boiling point, or even in the presence of the lubricating oil itself. While temperatures up to about -100 C. may be employed for short periods of time, it is advisable to employ lower reaction temperatures on the order of 25-60 C. in order to avoid excessive polymerization or loss of hydrogen sulfide from the product.

Theoretically, the condensation of equimolecular ratios of formaldehyde and dialkyldithiophosphoric acid should yield a simple methylol derivative; however, considerable quantities of a compound of somewhat higher molecular weight also appear to be formed. When the ratio of formaldehyde to dialkyldithiophosphoric acid is reduced, the amount of high molecular weight material in the product appears to increase; therefore this is probably a bis-compound. The two types of compounds possess about the same antioxidant properties for lubricating oils, and usually the entire condensation product is employed for this purpose. It should be understood, therefore, that the invention includes any oilsoluble antioxidant for hydrocarbon or vegetables and the like obtainable by condensing from 0.5 to 1 or more mols .of formaldehyde or a formaldehyde-yielding substance with one mol of a dialkyldithiophosphoric acid, regardless of the exact chemical constitution of the reaction product or products obtained.

Any 0,0-dialkyldithiophosphoric acid may be used in carrying out the process of our invention. Where antioxidants having a very high percentage of combined phosphorus and sulfur are desired it is preferable to employ a dialkyldithiophosphoric acid in which the alkyl groups are of relatively low molecular weight, such as methyl, ethyl, propyl or butyl radicals. Dialkyldithiophosphoric acids of somewhat higher molecular weight may be used, such as diamyl, dihexyl,- dicyclohexyl or dioctyldithiophosphoric acids, where products having a higher degreeof oil-solubility are desired. In some cases mixed diesters of dithiophosphoric acid are of value such as the methyl octyl ester or the ethyl hexyl ester. Dialkyldithiophosphoric acids of even higher molecular weight may also of course be used if desired, such as the didecyl, didodecyl, ditetradecylor dioctadecyldithiophosphoric acids. In general, therefore, the aliphatic radicals of the dialkyldithiophosphoric acid may contain from '1 to 18 or more carbon atoms and may be the same or different, and'the radicals of from 3 to 18 carbon atoms may be either straight or branched-chain radicals. For some purposes, as where extreme pressure lubricant properties are desired in the oil, these aliphatic radicals may be substituted by chlorine or other halogen atoms. Dithiophosphoric acid esters of unsaturated aliphatic or cycloaliphatic alcohols may also be employed.

The quantity of antioxidant to be used in the lubricating oil may vary from very small proportions on the order of 0.1% to relatively large quantities up to l-% or greater, the smaller amounts usually being employed when a compounded lubricant containing other additives is being prepared and the larger quantities being generally used for special purposes, as for extreme pressure lubricants. Ordinarily in automobile engine lubricating oils, quantities on the order of 0.2% up to about 2% are employed. Our novel antioxidants are compatible with all of the commonly used detergents, stabilizers and other ingredients of compounded oils and may be used in conjunction with smaller or larger quantities of aliphatic or aromatic sulfonates such as calcium petronate, alkylphenol sulfides such as p-p'-dibutvl-, diamyl-, or dioctyl phenol monoor polysulfides and their metal salts. metal salts of oxygen or sulfur-containing acids of phosphorus such as any of those described in the three patents referred to above, and the like. These and other additives are usually employed in the oils in quantities of about 0.1-5%, most commonly in quantities of about 0.5-2%, in admixture with the antioxidants of the present invention.

The invention will be further illustrated in greater detail by the following specific examples. It should be understood, however, that although these examples may describe in detail some of the specific features of the invention, they are given primarily for purposes of illustration and the invention in its broader aspects is not limited thereto.

Example 1 One hundred seventy-five grams (2.16 mols) of 37% formalin was added to 411 grams (2.01 mols) of undistilled diethyl dithiophosphoric acid with stirring in 1% hours. By the time the addition was half completed a considerable lightening of its color was apparent. The maximum temperature of the reaction was 41 C. when the addition was approximately complete. The mixture was stirred for 16 hours at room temperature. The product was put in a separatory funnel, the water layer removed and the organic layer washed once with water, both aqueous layers being discarded. The organic layer was then washed with an excess of 5% potassium carbonate solution. The aqueous layers from the carbonate wash-were acidified with concentrated hydrochloric acid and the oil which separated was recovered. This oil was washed with water, dried over calcium sulfate and stripped of volatile matter on asteam bath .under reduced pressure.

The weight of thisoil was 207 grams. It WaSZQr' light tan liquid which was found on analysis to contain 30.3% of sulfur and 16% of phosphorus.

The potassium carbonate-insoluble material from the separation procedure was washed with water, dried with calcium sulfate and stripped under reduced pressure on a steam bath. The yield was 150 grams of a liquid which was darker than the carbonate-soluble fraction. It contained 30.1% of sulfur and 14.1% of phosphorus.

Example 2 Twenty-one grams of 37% formalin was added slowly and with stirring to 105 grams of a 75% solution of dimethyl dithiophosphoric acid in benzene. This mixture was warmed on a steam bath for two hours. Benzene was added and the organic layer washed three times with water. The product was dried with plaster-of-Paris, filtered and stripped under reduced pressure at C. The product, weighing 24 grams, was a light colored liquid.

Example 3 To grams of a 75% solution in benzene of crude dimethyl dithiophosphoric acid was added 42 grams of 37% formalin with stirring in 10 minutes. The internal temperature increased to a maximum of 60 from heat of reaction. The product was dissolved in ethylene dichloride, washed twice with water and dried over sodium sulfate. Volatile materials were removed at 95 C. and 8 mm. pressure to yield 60 grams of product, which was a light-colored liquid.

Example 4 To 50 cc. of a 36% aqueous formaldehyde solution there was added slowly 25.5 grams (0.1 mol) of di(2 chloroethyl)dithiophosphoric acid of the formula The reaction mixture was agitated and maintained at 25-30 C. while the acid was being in troduced and for an additional 16 hours. It was then allowed to stand for the separation of a lower oily layer which was drawn off and stripped of unreacted material by heating under reduced pressure. The product, weighing 27 grams (95% yield) was a light yellow liquid having a specific gravity at 25 C. of 1.5193 and a phosphorus content of 8.68. It was readily soluble in SAE 20 lubricating oil.

Example 5 The di-cyclohexyl ester of dithiophosphoric acid was prepared by reacting cyclohexanol with P2S5, using a 4:1 molar ratio.

To 147 grams (0.5 mol) of this di-cyclohexyl dithiophosphoric acid there was slowly added 16.6 grams (0.2 mol) of 36% aqueous formaldehyde solution while maintaining the temperature below 40 C. and the mixture was stirred overnight without heating. The reaction product was diluted with twice its volume of ether and washed three times with 5% aqueous sodium carbonate solution and then once with water. After drying in a desiccator the ether solvent was evaporated and the product stripped of volatiles by heating under reduced pressure. The residue was a dark green viscous material.

Upon repeating the reaction using an equimolecular quantity (41.5 grams) of formalin, a darkcolored oily product was obtained as a lower layer in the reaction mixture. This layer was washed by agitating it with aqueous sodium carbonate solution, neutralizing with acid, and separating .the non-aqueous fraction which was then washed with water. After heating under reduced pressure to remove volatiles the product was obtained as a dark liquid which was readily soluble in lubricating oil.

Example 6 Eighty-one grams of 0,0-didecyldithiophosphoric acid (neutralization equivalent 440) were mixed with 20 grams of 37% aqueous formaldehyde solution and the mixture was stirred for 7 hours at room temperature and then allowed to stand overnight. The resulting oily layer was separated and washed several times with saturated sodium chloride solution and dried over anhydrous calcium sulfate. The product was a dark brown oily liquid that was easily soluble in lubricating oil.

' Example 7 Representative products of the present invention were evaluated as lubricating oil antioxidants by the Underwood oxidation test. In this test 1500 cc. of oil are used and 0.04% of iron naphthenate, based on the F6203 equivalent, is added as an oxidation catalyst. The additive under test is dissolved in the oil, which is then heated for 10 hours at 325 F. in an open container providing free circulation of air while portions of the oil are sprayed continuously against two freshly sanded alloy bearings.

An SAE '30 Mid-Continent base oil containing 0.5% of the 0,0-dia1kyldithiophosphoric acidforxnaldehyde condensation product together with 0.04% of iron naphthenate was used in the tests. Another sample of the same oil, containing the iron naphthenate but no antioxidant, was also tested as a control. Silver-cadmium alloy bearings were used in all the tests and were weighed before and after exposure to the oil. The results were as follows:

Product of Weight loss, mg. Control 1097 Example 1 (two products mixed) None Example 5 6 Example 6 8 EDWIN O. HOOK. PHILIP H. MOSS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,063,629 Salzberg et al Dec. 8, 1936 2,167,867 Benning et al Aug. 1, 1939 2,346,155 Denison et.a1 Apr. 11, 1944 2,372,244 Adams et a1. Mar. 27, 1945 2,485,573 Craig et a1. Oct. 25, 1949 

1. THE CONDENSATION PRODUCT OF ONE MOL OF FORMALDEHYDE WITH FROM ONE TWO TWO MOLES OF AN O,O-DIALKYLDITHIOPHOSPHORIC ACID ADIESTER. 